Britons living in the north of England have been warned that they must boil tap water before using it because the water is contaminated with a potentially serious stomach bug.
As many as 108,000 homes in Northampton, Daventry and nearby villages are affected and 250,000 people face weeks of boiling tap water after the stomach bug Cryptosporidium was found following tests by Anglian Water.
Cryptosporidium can cause severe illness which lasts for weeks and 21 schools have already been closed because they did not have bottled water.
The public have been warned in the media to boil water for cooking, drinking and cleaning teeth and vans with loudspeakers have also been sent out to warn people in the affected areas.
Supermarkets have had to ration sales of bottled water as customers began panic buying and cleared the shelves of stocks.
Anglia Water is investigating the cause of the contamination at a treatment works near Pitsford reservoir, north of Northampton and say even when the source is confirmed the entire system will need flushing out which could take weeks.
The Cryptosporidium parasite is present in the environment at low levels at all times and is common in farm and domestic animals, it is passed on through their manure or their corpses.
Greater concentrations occur when heavy rain washes it into the ground, rivers and lakes but it is usually destroyed during the water treatment process.
Anglian Water says it routinely carries out about 1,000 water quality tests each day and while Cryptosporidium is a naturally occurring bacteria if it enters the human body it can cause an infection called Cryptosporidiosis (KRIP-toe-spo-rid-ee-OH-sis) which can be severe.
Cryptosporidiosis is most common in young children but can affect anyone and the elderly and those with weakened immune systems are most likely to be seriously affected - there is no specific treatment.
While it remains unclear how far into the water supply the bug has travelled, the Health Protection Agency says to date there were no confirmed cases of Cryptosporidium in the area.
Anyone with symptoms such as diarrhoea, stomach pains, dehydration, weight loss and fever, are advised to contact their GP.
Cryptosporidium has an incubation period of five to seven days and it may be a week before it is clear if anyone has been infected.
Pets' drinking water should also be boiled but the tap water is considered safe for showers and baths, washing and toilet flushing.
An outbreak of Cryptosporidium in the North Wales water supply in 2005, sickened 231 people and 70,000 homes were forced to boil their water for a two month period. Welsh Water was eventually fined £60,000.